The present invention relates to a light beam scanning device, and more particularly relates to a small-sized light beam scanning device, by which deflection-scanning is conducted on a beam of light sent from a laser beam source, and an image formation cylindrical lens, wherein a rotational polygonal mirror is used for the scanning operation.
A rotational polygonal mirror 4 is as shown FIGS. 1(b) and 1(c) generally used for a light deflection means in an optical system of a light beam scanning device such as a laser beam printer and other devices as shown in FIG. 1(a). In the case where a rotational polygonal mirror is used in order to reduce the restriction of the allowance of the tilt of surface on a reflecting surface 41, the following conditions are commonly maintained:
In the secondary scanning direction, a reflecting surface 41 of the rotational polygonal mirror 4 and the surface 7 to be scanned are approximately set in a geometrical-optical conjugate relationship, and a light beam condensed on the surface of the secondary scanning direction is incident upon a position close to the aforementioned reflecting surface. However, as the rotational polygonal mirror is rotated, the position of the reflecting surface is moved. As a result, the aforementioned conjugate relationship can not be maintained. Therefore, an image formation point on the surface of the secondary scanning direction is shifted, so that an image surface curvature is caused with respect to the image formation point on the surface of the secondary scanning direction. Further, as illustrated in FIG. 1(a), a beam of light incident upon the aforementioned reflecting surface 41 is sent in the lateral direction forming an angle .alpha. with respect to an optical axis of the image formation lens system on a developed surface including the scanning line and an optical axis of the image formation lens system. Accordingly, a rotational center 5 of the rotational polygonal mirror is not located on the optical axis of the image formation lens system, so that the movement of the aforementioned reflecting surface is not symmetrical to the optical axis of the image formation lens system. For this reason, in the case where a lens system symmetrical to the optical axis of the image formation lens system is used, it is difficult to appropriately correct the image surface curvature of the image formation point on the secondary scanning direction all over the image formation regions of the surface to be scanned while consideration is given to the quantity of movement of the reflecting surface which is not symmetrical with respect to the optical axis of the image formation lens system.
Further, in the case where the required resolution is low, practical problems are not caused even when a lens system symmetrical with respect to the optical axis is used as the image formation lens system. However, when a large scanning angle is used in order to reduce size of the scanning device, or as the required resolution becomes higher, the curvature of the image surface is increased which remains in the case where the lens system symmetrical with respect to the optical axis is used as the image formation lens system.
A scanning device in which the image surface curvature is corrected overall the surface to be scanned is disclosed, for example, in Japanese Patent Publication No. 49408/1991. In this scanning device, a deformed cylindrical lens shown in FIG. 15 is used, in which the radius of curvature rh in a section of the secondary scanning direction is determined as a function of the distance h from the optical axis of the image formation lens system. Therefore, this deformed cylindrical lens can provide a wide field angle which can not be provided by conventional cylindrical or toric lenses. However, a change of the radius of curvature rh(+) on the "+" image height side of the cylindrical lens (the side in which the rotational center of the rotational polygonal mirror is located with respect to the optical axis of the image formation lens system) and the radius of curvature rh(-) on the "-" image height side (the side in which the rotational center of the rotational polygonal mirror is not located with respect to the optical axis of the image formation lens system) is symmetrical with respect to the optical axis of the image formation lens system. Therefore, this scanning device can not correct a fluctuation of the image formation position on a surface of the secondary scanning direction which is generated in accordance with the fluctuation of a reflecting surface position when a rotational polygonal mirror is used as a deflecting system.
According to a scanning device disclosed in Japanese Patent Publication Open to Public Inspection No. 23313/1990, the radius of curvature of a cylindrical lens on the secondary scanning direction is made to be non-symmetrical with respect to the optical axis of the image formation lens system and is monotonously increased in order to satisfactorily correct the fluctuation of the image formation position on the secondary scanning direction which is generated in accordance with the fluctuation of a reflecting surface position when a rotational polygonal mirror is used as a deflecting system. However, in this case, a lateral magnification of the image formation lens system is not less than 3 and not more than 5. As described above, the magnification is high. Accordingly, it is required to manufacture the device with a high degree of accuracy, with a resulting increase in cost. Further, since the lens, including the non-symmetrical surface, is made of glass, it is difficult to manufacture the non-symmetrical surface. Therefore, the manufacturing cost is raised from the view point of workability. Consequently, the above scanning device is not viable for mass production in terms of the cost.
In the scanning device disclosed in Japanese Patent Publication Open to Public Inspection 60608/1992, the shape of a cylindrical lens on the section of the primary scanning direction is non-symmetrical with respect to an optical axis of the image formation lens system, and a change of the radius of curvature of the cylindrical lens on the section of the secondary scanning direction is symmetrical with respect to an optical axis of the image formation lens system. In this case, the scanning angle is .+-.20.degree., which is small, and the size of the overall scanning device increases.